1. Field of the Invention
The present invention relates to a drier using gas combustion, and more particularly, to an apparatus for controlling the gas valves of such a drier.
2. Discussion of the Related Art
Generally speaking, a laundry drier uses an energy source, such a gas or electricity, to heat a conductor such as a coil, which in turn radiates heat to the surrounding air. The thus-heated air is circulated inside the laundry drier to dry the laundry. FIG. 1 schematically illustrates a gas valve controller of a gas-combustion-type laundry drier according to a related art.
Referring to FIG. 1, a microcomputer 10 outputs an ignition signal according to a user command for generating heated air, to control a normally open start relay 11, which is closed to supply power through a terminal L1 to an igniter 12 for igniting a gas-and-air fuel mixture. When a flame of the igniter 12 is detected, a normally closed flame detection switch 13 is opened. Meanwhile, a normally closed thermostat switch 14 is opened when an overheating condition is detected. To control a fuel supply, first and second valve coils 16 and 17 for controlling respective gas supply valves (not shown) are commonly supplied with a rectified voltage, via a rectifier bridge 18 for rectifying the power applied to the igniter 12 and the voltage felt from the common electrodes of the flame detection and thermostat switches 13 and 14. A pair of photo-couplers 15a and 15b is employed for transferring to an input port of the microcomputer 10 a flame detection signal from one of the flame detection switch 13 and the second valve coil 17, respectively.
The operation of the circuit of FIG. 1 will now be explained with reference to the timing diagram of FIG. 2.
The microcomputer 10, in response to a user command to start the laundry drier, closes the start relay 11 and thereby transfers power to the igniter 12 for heating to begin. Simultaneously, the first valve coil 16 turns on its valve to supply the gas, using a rectified applied from the rectifier bridge 18. At this time, the flame detection and thermostat switches 13 and 14 are both closed. Subsequently, after the igniter 12 is activated, for as much as twenty to thirty seconds to ignite the gas, the flame detection switch 13 detects a flame and is thus switched to the open state, whereupon the igniter is turned off. As the igniter 12 is turned off, a flame detection signal is applied to the microcomputer 10 and second valve coil 17 through the photo-coupler 15b. The valve of the second valve coil 17 is thus turned on, and gas combustion is normalized.
After a continuation of the above state of gas combustion, an internal temperature of the gas drier rises, exceeding a set temperature limit according to a controlled characteristic of the thermostat switch 14, which opens to prevent overheating. In doing so, the power supply of the terminal L1 is cut off, thus turning off the valves of first and second valve coils 16 and 17 and in turn stopping the combustion. As the internal temperature drops, the thermostat switch 14 is again closed, to regenerate the gas combustion state.
To close the flame detection switch 13 at the time of regenerating the gas combustion state, however, which has been in an open state, a time longer than the above twenty to thirty seconds is required and is typically as much as thirty to forty seconds. That is, after the thermostat switch 14 has been closed, there is a time lag (gap) of about ten seconds, where the flame detection switch 13 maintains its open state. With the thermostat switched 14 in the closed state as above, a terminal N is connected to the terminal L1 through the rectifier bridge 18 so that the first valve coil 16 is powered and its gas supplying valve remains open. The gas supplying valve of the second valve coil 17 is also open, since the flame detection signal of the flame detection switch 13 is present at the second valve coil through the photo coupler 15b. Accordingly, with both gas supplying valves thus open.
Where the time required for heating the igniter 12 to re-ignite the gas is insufficient, whereby ignition fails while gas is discharged (leaked) from the gas valves of the first and second valve coils 16 and 17. That is, the gas leaks from the time that the gas valves are still on to the time that the igniter 12 restarts its heating process. Therefore, in the process of resuming a state of combustion that has been stopped due to an overheating condition, i.e., after an earlier state of combustion, the gas drier according to the related art fails to ignite because of an operation delay of the elements. Moreover, the interim condition of gas leakage is dangerous.